To sustain growth and differentiation, embryos must create a functional circulatory system during early development. Little is known about the tissue interactions that may be required to induce the formation of angioblasts (vessel endothelial precursors). The investigator proposes first to examine the spatial and temporal expression pattern for cells that express an early vascular endothelium-specific marker, xEGR1 (an Endothelium G-Protein coupled Receptor), and a putative receptor tyrosine kinase homolog of vascular-specific mouse genes flk-1 and flt. She also proposes to determine the region of the frog embryo that gives rise to angioblasts by creating a precise fate map of cells that later express xEGR1, and further to examine the appearance of developing vacular tissue with scanning electron microscopy. Second, she proposes to examine tissue interactions that may be important in angioblast induction by using tissue co-cultures and treatment of undetermined tissues with various known mesoderm inducers or agents known to disrupt mesoderm specification. Finally, she proposes to investigate whether xEGR1 has a direct role in angiogenesis/vasculogenesis by forcing ectopic expression of xEGR1 and by treating cells with agonists and antagonists of xEGR1 protein.